From the ticket conditions on the
FGW▸ booking site:
Availability:
Tickets are valid ONLY on the date and train service(s) shown on the ticket(s). Where applicable, you must travel in the class and reserved seat(s) shown on the ticket(s).
This appears to have become a bit of a technicality as far as the reserved seat element is concerned ... so just sayin' ...
Twice, in the last eight years, I've been actually asked to sit in the seat allocated to me - and both times on fairly empty trains. Normally, the
TM‡'s view has been 'as long as you are on the right train and in the correct class of accommodation....'
Most of the time, I am not really that bothered, but yesterday, my journey from Hereford to Paddington was the first time I had been on that line and I wanted to enjoy it properly without too many other distractions.